Om Mani Padme Hum

The mantra Om Mani Padme Hum (literally: “Aum, to the Jewel in the Lotus, hum) is recited by Tibetan Buddhists to invoke Chenrezi, the Bodhisattva of Compassion. Repeating this mantra accumulates merit and eases negative karma; meditating upon it is believed to purify the mind and body. Spinning prayer wheels, physical or digital, are believed to confer the same benefit as speaking the mantra. It is often recited with the aid of a mala (string of prayer beads.)

Tibetan Buddhists believe that saying the mantra (prayer), Om Mani Padme Hum, out loud or silently to oneself, invokes the powerful benevolent attention and blessings of Chenrezig, the embodiment of compassion. Viewing the written form of the mantra is said to have the same effect — it is often carved into stones, like the one pictured above, and placed where people can see them.
Spinning the written form of the mantra around in a Mani wheel (or prayer wheel) is also believed to give the same benefit as saying the mantra, and Mani wheels, small hand wheels and large wheels with millions of copies of the mantra inside, are found everywhere in the lands influenced by Tibetan Buddhism.
It is said that all the teachings of the Buddha are contained in this mantra: Om Mani Padme Hum can not really be translated into a simple phrase or sentence.
It is appropriate, though, to say a little about the mantra, so that people who want to use it in their meditation practice will have some sense of what they are doing, and people who are just curious will understand a little better what the mantra is and why it is so important to Tibetan Buddhists. We begin in the next section with some information about the mantra itself.
(from: Dharma Haven)